1. Field of the Invention
The present invention relates to the field of satellite receivers, and more specifically to satellite to home direct broadcast receiver systems for reception of digitally modulated broadcast information.
2. Prior Art
Digital television is digitally modulated when broadcast over satellite systems using phase shift keyed modulation schemes. The signals are typically received at Ku-band or C-band via a satellite dish antenna. The signals are first amplified and then downshifted to a predetermined frequency band, typically in the L-band, in the range 950 MHz to 2150 MHz. This function is performed within the Low Noise Block (LNB), a subsystem contained with the satellite dish assembly. The signal is then sent via a coaxial cable to the set-top box unit located indoors. In the prior art, the signal received at the set-top box is downshifted to a predetermined intermediate frequency for amplification, bandpass filtering to eliminate adjacent channels and other functions such as automatic gain control, etc., with a subsequent or second down conversion to baseband and recovery of the PSK (phase shift keyed) modulated data.
A specific representative system for reception of the direct broadcast satellite (DBS) signal and extraction of the digital data in the signal is shown in FIG. 1. The signal delivered to the set-top box through cable 20, typically in the frequency range of 950 MHz to 2150 MHz, is amplified through one or more amplifiers 22, 24 and/or 26 and passed through an image filter 28 to an L-band tuner. Typically, gain control is provided in one or more amplifiers amplifying the received signal, such as by way of a variable gain amplifier 24 or a fixed gain amplifier 26 operating into a variable resistive attenuator 30.
The L-band tuner, controlled by external voltage controlled oscillator (VCO) 32, in turn referenced to a crystal controlled phase locked loop 34, downshifts the received signal by mixer 36 to the intermediate frequency, with the signal then passing through a surface acoustic wave (SAW) filter 38 to isolate the channel of interest. This filter is a fixed bandpass filter, the channel falling within the bandpass of the SAW filter being selected by the specific frequency chosen by the oscillator 34 to determine the amount of downshifting to put the desired channel within the bandpass window of the SAW filter.
The output of the SAW filter is then passed to a MAX2101 integrated circuit manufactured by Maxim Integrated Products, Inc., assignee of the present invention. The MAX2101 is controlled by a phase locked loop 40 and tank circuit 42 to mix the output of the SAW filter with in-phase and quadrature components of the output of the tank circuit by mixers 44 and 46 to provide in-phase (I) and quadrature (Q) components of the signal at base band. The I and Q signals are passed through low pass filter 48 and then converted to digital form by analog to digital converters 50 for demodulation by digital signal processor (DSP) 52, which also typically provides an automatic gain control (AGC) signal to control the gain of one or more amplifiers in the system so that the amplitude of the digitized I and Q signals received by the DSP are as expected by the DSP.
Another example of the use of an intermediate frequency may be found in U.S. Pat. No. 5,325,401. The practice of shifting a received frequency band, first to an intermediate frequency for such purposes as further amplification, automatic gain control, station or channel isolation, etc., is an old practice dating back to the early days of radio. It has the advantage of requiring minimal tunable components and of making each channel or station have substantially identical characteristics at both the intermediate frequency and at baseband. However, it has the disadvantage of complexity in the number and nature of components required.